Decoding the spectra of SDSS early-type galaxies: new indicators of age and recent star formation

Abstract

We apply Principal Component Analysis (PCA) to a sample of early-type galaxies from the Sloan Digital Sky Survey (SDSS) in order to infer differences in their star formation histories from their unresolved stellar populations. We select a z < 0.1 volume-limited sample comprising � 7000 early-type galaxies from SDSS/Data Release 4. Out of the firs t few principal components (PC), we study four which give information about stellar populations and velocity dispersion. We construct two parameters (η and ζ) as linear combinations of PC1 and PC2. The four components can be presented as “optimal filt ers” to explore in detail the properties of the underlying stellar populations. By comparing various photo-spectroscopic observables ‐ including NUV photometry from GALEX ‐ we find ζ to be most sensitive to recent episodes of star formation, and η to be strongly dependent on the average age of the stellar populations. Both η and ζ also depend on metallicity. We apply these optimal filters to composite spectra assembled by Bernardi et al. The distribution of the η component of the composites appear to be indistinguishable between high and low density regions, whereas the distribution of ζ parameters have a significant skew towards lower values for g alaxies in low density regions. This result suggests that galaxies in l ower density environments are less likely to present weak episodes of recent star formation. In contrast, a significant number of galaxies from our high density subsample ‐ which includes clusters (both outer regions and centres) and groups ‐ underwent small but detectable recent star formation at high metallicity, in agreement with recent estimates targeting elliptical ga laxies in Hickson Compact Groups and in the field (Ferreras et al.).